Radio Make.Uniden
Radio Model.Grant
PCB number.PC-409AC
PLL number. MB8719
Hi I have just acquired a Uniden Grant (basically a president Grant) Like brand new, with pc board number PC-409AC, But it has a problem i do not have any audio rx or tx on usb, yet am & lsb work 100% perfect.I can hear a bit of a hiss on rx usb with volume full up,but not registering any signal level on the s meter & totally nothing on t.x!, Any idea's folkz??? as i really want to have it working 100% as even though its just 40 channels on midband i want to band it up,Thanks beforehand,Shaun
There can't be that much wrong with it if it's only USB that's not working, should be easy enough to find the fault with the circuit diagram (see link below):
Hi Brian , i already tried the mode switch & that seems fine as i swapped the lsb & usb wires around & it still did'nt work on usb.I already have the circuit diagram & block diagram, but i am not that well electonically minded in knowing how to read or understand the different componants/stages etc,hence why im here to ask,cheers,Shaun
DIRTY HARRY wrote:Hi Scott would the Balanced modulator be IC3 which is a AN612 ?
Apologies to Scott for jumping in and answering, but yes that's correct IC3, the AN612 is the BM. I doubt very much that this is the problem, otherwise it wouldn't be working on LSB.
Brian Morris wrote:Did you test the diodes when you removed them, or replace them with new diodes?
Brian.
nope not yet Brian, i hav'nt looked at their values yet on the schematic & i have only just started looking at this rig ,so i have no replacements just yet.
They are probably just standard silicon diodes, in which case a 1n4148 will be fine as a replacement. Definitely worth cheching that L25 isn't open circuit as well.
Brian Morris wrote:They are probably just standard silicon diodes, in which case a 1n4148 will be fine as a replacement. Definitely worth cheching that L25 isn't open circuit as well.
Brian.
Hi again Brian mmm i see maplin actually do stock something that i may need for once, as they actually stock 1n4148's.
Is there a way i can test the d33 d37 & d39 diodes & how do i do this,
i would like to test them rather than a wasted journey to craplins.
I have also tried swapping L25 with L26 & it maade no difference,
Cheers, Shaun
If you have a continuity tester, put both probes accross the diode, then swap them round, it should conduct one way but not the other way.
These days most multimeters have a diode test setting on them, in which case the meter should show 0.6 to 0.7 volts one way round, and nothing the other way round on a good diode.
If you're going to be fixing rigs on a regular basis, a multimeter is a must. They're cheap as chips these days.
If you have a continuity tester, put both probes accross the diode, then swap them round, it should conduct one way but not the other way.
Brian.
Right Brian, i have tested D33,D37,D38 & D39 All only conduct one-way like you said at 0.6 volts or just above, so thats one we can cross off the list.As for can L28 I had already resoldered that.
Ok Shaun, rather than guesssing, I think we'll have to start fault finding and do some voltage measurements; let's start with the mode switch, what voltage do you get on the USB tag when in USB mode, and what voltage do you get on the LSB tag when in LSB mode?
Hmm, seems a bit strange that, if anything I would have expected to see 8 Volts on AM and LSB since you say that AM and LSB are working properly.
You should see 8 Volts on the appropriate tag for the selected mode, if you don't then either the switch is knackered, or something has gone short circuit and is dragging down the voltage for that mode. Anyway, you've said that you already tested the switch by swapping the wires around.
By the way I think L25 is the USB choke and L26 is the AM choke.
I am running out of ideas Shaun, the last thing I can suggest is that maybe one of C125, C126, or more likely CT2 have gone short circuit and this is dragging down the 8 Volts for USB mode. If any of the caps show continuity on your meter then they're buggered.
Hi Brian , yep im also getting confused, especially with those voltage readings off the pins, i just tested C125 & C126 & they are alright to, im thinking of letting a decent tech to look at it as its doin my head in mate Shaun.
I have just replaced balanced modulator AN612 & still no success, Dam! wish i could get this old bird to work on USB as its in exceptionally good physical condition
If you have a frequency counter with a input for a probe you could check the frequencies to see if the loop and I.F. are working, if they are that would rule out a LOT of things.
Hi Scott ,, i do have a thurlby thandar lab counter with a probe leads, i know its not 100 % accurate but works , So i could test the i,f & loop frequencys if you could please tell me the test points & procedure to carry this out,thanks beforehand,Shaun
The alignment procedure and info is on the main site in the alignments section, I suggest that you look it up and try to go through it, and let us know if anything does not appear to be correct with your radio.
I have just acquired a Uniden Grant (basically a president Grant) Like brand new, with pc board number PC-409AC, But it has a problem i do not have any audio rx or tx on usb, yet am & lsb work 100% perfect.I can hear a bit of a hiss on rx usb with volume full up,but not registering any signal level on the s meter & totally nothing on t.x!, 
Any idea's folkz??? as i really want to have it working 100% as even though its just 40 channels on midband i want to band it up,Thanks beforehand,Shaun 
, i already tried the mode switch & that seems fine as i swapped the lsb & usb wires around & it still did'nt work on usb.I already have the circuit diagram & block diagram, but i am not that well electonically minded in knowing how to read or understand the different componants/stages etc,hence why im here to ask,cheers,Shaun 
as its in exceptionally good physical condition 
